1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
A method for forming an image using a laser is widely used for obtaining a high-definition and high-quality image in an image recording field using electrophotographic technology. In the case of the electrophotography, typically, a latent image is formed on a photosensitive drum by rotating the drum in a sub-scanning direction while scanning the surface thereof with laser beams using a polygon mirror in an axial direction (a main-scanning direction) of the drum. In the field of electrophotography, there is a demand for forming a high-density image at high speed. Although high density and high speed have a trade-off relationship, it is required to achieve high speed and high density image formation.
One approach to achieve the high speed and high density image formation is to rotate the polygon mirror at a high speed. However, the high-speed rotation of the polygon mirror leads to increase of noises, increase of power consumption, and degradation of durability. A multibeam scanning device that does not have these problems has been put into practice. Specifically, the multibeam scanning device uses a light source unit in which a plurality of edge emitting laser diodes is combined (see Japanese Patent Application Laid-open No. 2005-250319), a one-dimensional edge emitting laser diode array, or a two-dimensional laser diode array.
In the light source unit in which a plurality of edge emitting laser diodes is combined, a manufacturing cost is low because a general-purpose laser diode can be used. However, it is difficult to keep a relative position relation stable between the laser diodes and a coupling lens with respect to a plurality of beams, and thus a scan-line interval formed on a scanning surface by the beams becomes uneven. Moreover, because it is difficult to use extremely large number of light sources in this method, ultrahigh density and ultrahigh speed image formation cannot be achieved.
The one-dimensional edge emitting laser diode array can have a uniform scan-line interval. However, power consumption of elements increases. When the number of beams is too large, a deviance of the beams from the optical axis of the optical element of an optical system increases, and thus an optical characteristic degrades.
On the other hand, a surface emitting laser such as a vertical-cavity surface-emitting laser (VCSEL) is a semiconductor laser that emits light in a direction perpendicular to a substrate thereof, and thus can be easily integrated in a two-dimensional manner. Furthermore, the surface emitting laser consumes power that is about one digit less than the edge emitting laser. In this way, the surface emitting laser is advantageous in integrating more light sources in a two-dimensional manner.
A writing optical system for scanning light beams using a polygon mirror has been disclosed in, for example, Japanese Patent Application Laid-open No. 2004-287292, in addition to Japanese Patent Application Laid-open No. 2005-250319. In the surface emitting laser, light sources are easily arrayed in a two-dimensional manner and thus the number of beams can be increased compared with the edge emitting laser diode.
However, because a distance in the sub-scanning direction between beams at both ends in a single scanning gets longer as the number of beams is increased, a deviance of the scan-line interval (hereinafter, a sub-scanning beam pitch) becomes large when the magnification of the optical system of the optical scanning device deviates from a desired value.